The present invention relates generally to the preparation of silicon single crystals grown by the Czochralski process. The invention particularly relates to eliminating dislocations within the neck before the body of the silicon single crystal is grown.
Single crystal silicon which is the starting material for most processes for the fabrication of semiconductor electronic components is commonly prepared with the so-called Czochralski process. In this process, polycrystalline silicon ("polysilicon") is charged to a crucible and melted, a seed crystal is brought into contact with molten silicon and a single crystal is grown by slow extraction. As crystal growth is initiated, dislocations are generated in the crystal from the thermal shock of contacting the seed with the melt. The dislocations are propagated throughout the growing crystal and multiplied unless they are eliminated in the neck region between the seed crystal and the main body of the crystal.
The conventional Dash method of eliminating dislocations within silicon single crystals involves growing a neck having a small diameter (2 to 4 mm) at a high crystal pull rate (as high as 6 mm/min) to completely eliminate dislocations before growth of the body of the crystal. The dislocations are eliminated when the neck (known as a Dash neck) is grown to a length of up to 100 mm.
After dislocations are eliminated in the neck, its diameter is enlarged until the desired diameter of the main crystal body is reached. The body of the crystal is pulled from the melt until most of the melt is depleted, the tapered tail end of the crystal is formed, and the crystal is removed from the crystal pulling apparatus.
The neck, which is the weakest point of the crystal, can fracture during crystal growth, causing the body of the crystal to drop into the crucible. The impact of the crystal ingot and splashing molten polysilicon can destroy the crucible, susceptor, and heater, render the polysilicon melt unsalvageable, and present a serious safety hazard. Conventional 200 mm diameter crystals having a Dash neck are typically grown to a weight of 100 kg or less to minimize stress fracture of the neck.
Attempts have been made to minimize equipment and raw material losses and safety hazards from neck fractures by increasing the diameter of the neck. Japanese Kokai No. 05-43379 describes a method of eliminating dislocations while forming a neck having a diameter greater than that of a Dash neck. Dislocations are removed when the neck is pulled at a rate ranging from 4 mm/min to 6 mm/min and maintained at a constant diameter ranging from 4.5 mm to 10 mm. When the neck diameter exceeds 10 mm, dislocations are said to be difficult to eliminate.
It is generally accepted within the industry that dislocations cannot be consistently removed from necks having diameters exceeding 10 mm. The necks of most silicon single crystals continue to be grown according to the Dash technique even though the weight of a crystal body that can be supported by the Dash neck is limited.
Others have attempted to reduce neck fractures by providing additional support for the crystal body. U.S. Pat. No. 5,126,113 describes an apparatus for supporting a single crystal ingot as it is grown. Dislocations in the crystal are eliminated by growing a small diameter neck by the Dash method. A large diameter bulge is then grown beneath the Dash neck before the start of the conical section of the crystal body. Mechanical grips engage the recess beneath the bulge to support the body as it is grown. When the grips hold the crystal, they may disturb the steady crystal growth operating conditions and may also break the Dash neck.
There is a need for an improved process for eliminating dislocations within the neck of a single crystal such that larger diameter, zero dislocation (i.e., dislocation-free) single crystals can be produced without substantial equipment damage, loss of raw materials, safety hazards and reduced throughput and yield.